Vacuum distillation is a well-known process for separating chemical components on the basis of differences in vapor pressure.
Distilling under vacuum has several advantages over distilling at atmospheric or superatmospheric pressures, because of the lower temperatures at which the process can be conducted. Economical low-grade heat sources may be used. Safety is enhanced by the low temperature operation, and thermal degradation of the chemical components is minimized. Furthermore, leaching of impurities from the walls of the apparatus itself is minimized. Some of the best materials of construction, such as polyethylene, cannot be used at the higher temperatures but work well at the temperatures of vacuum distillation.
In order to protect product purity, the vacuum should be generated by liquid head rather than by vacuum pumps. However, the ceiling height required by towers for achieving middle-or high-vacuum is often not available.
A vacuum distillation apparatus which works well in areas of limited headroom was patented by Applicant in U.S. Pat. No. 4,444,623 and is incorporated by reference. A shortcoming of that apparatus is that the presence of air bubbles requires the distillate pump to be a positive displacement pump such as a gear pump, which will pump a mixture of liquid and gas or vapor. Less expensive centrifugal pumps cannot be used because the gas or vapor accumulates at the center of the impeller and blocks the flow of liquid.